ND3+ AM3+ ION INTERACTIONS WITH SULFATE ION AND THEIR INFLUENCE ON NDPO4(C) SOLUBILITY

The effects of Nd(III)/Am(III) complexation with sulfate were studied by 1) re-examining existing data for the Am-SO4 system using more adva nced aqueous electrolyte models valid to high concentration to obtain reliable thermodynamic data for SO42- complexes or ion interactions wi th Nd3+ and Am3+ and 2) conducting experimental solubility studies of NdPO4(c), an analog phase of AmPO4(c), a possibly important phase in h igh level nuclear wastes, in the presence of SO42- to test the newly d eveloped thermodynamic model and show the possible influence of sulfat e in a repository environment. The data showed that the increase in th e solubility of NdPO4(c) resulted primarily from the increase in ionic strength. Slightly higher observed Nd concentrations in the presence of sulfate, as compared with concentrations predicted at the experimen tal ionic strengths, resulted from the weak complexes or ion interacti ons involving Nd3+-SO42-. The Pitzer ion interaction parameters, appli cable to 0.5m sulfate, were obtained for Am3+-SO42- from a reinterpret ation of known solvent extraction data. These parameters are also cons istent with literature data for Am3+/Na+ exchange and solvent extracti on in, the presence of sulfate. When used for the analogous Nd3+-SO42- system to predict NdPO4(c) solubility in the presence of sulfate, the y provided excellent agreement between the predicted and the observed solubilities, indicating that they can be reliably used to determine N d3+ or Am3+ ion interactions with SO42- in all ground waters where SO4 2- is less than 05m.